This invention relates to radiation detection tubes for detecting beta type radiation. More particularly the invention relates to Geiger-Mueller tubes of the type having a thin mica window for admitting beta radiation. In still more particularity the invention relates to a means of reducing erroneous counts due to activation of the tube by ultraviolet light and arcing between the tube anode and window.
Geiger-Mueller tubes of the type to which this invention relates are well known and in general consist of a metallic cylinder closed on both ends. One end closure is a radiation transparent window usually of mica. An anode is formed by mounting a thin wire coaxially within the tube from the end opposite the window. A potential difference is maintained between the anode and case which functions as a cathode. The tube is filled with a gas that becomes ionized when atoms of the gas are struck by radiation particles such as beta rays. The potential difference between the anode and cathode causes the free electrons produced by the ionization to migrate to the positive anode and the positive ions to migrate to the cathode. The resulting change in charge on the anode and cathode is detected as a count of the radiation particles entering the tube.
One problem associated with this type of detector is erroneous counts due to activation of the tube by means other than beta particles. One means of activation that has been encountered is the activation of the tube by ultraviolet light near the x-ray region if allowed to enter the tube through the mica window. It has been found that means to prevent the entry of ultraviolet light into the tube has given rise to another means of tube activation and, additionally, threatens the useful life of the Geiger-Mueller tube. Coatings such as india ink and collodial graphited water solutions have been applied to the window and while they are effective in preventing the entry of ultraviolet light, they are electrically conductive, and it has been observed that electrical arcing can occur between the tube anode and the coating.
In order to make the detector sensitivity acceptable and to detect the particular particle type desired it is often necessary to place the free end of the anode wire in close proximity to the mica window, or due to manufacturing tolerances or errors the anode is located so close to the window that arcing occurs. Even when the conductive coatings are applied to the outer surface of the mica the potential difference between the anode and coating can be sufficiently high that the arc penetrates the mica to reach the conductive coating. It has been found that this arcing will saturate the tube and render it useless in measuring radiation sources. Constant arcing within the tube uses up the quenching gas and shortens the life of the tube.
Means to redirect the arc away from the window to another surface have been devised, such as that disclosed in Patent 2,452,524 for Protective Grid for Geiger Mueller Tubes to Metten; however, the arc is not eliminated but rather redirected to another surface and may still result in an erroneous count.
It can be appreciated that it is desirable to have a mica window that is not only opaque to ultraviolet light and transparent to the beta radiation to be measured, but one that also prevents arcing thereto. Additionally, it is highly desirable to provide /for a coating that can be applied to the windows of tubes, that have been improperly manufactured with their anodes so close to the window that arcing would occur if an electrically conductive coating were applied. In other words, those tubes to which opaque to ultraviolet light, conductive coatings have been applied, and which experience arcing because their anodes have been improperly positioned too close to the window during manufacture, can be salvaged by removing the conductive coating and applying the coating of this invention.